a) Field of the Invention
This invention relates to a process for preparing polymer emulsion particles having pores, useful as an additive for coating compositions used as paints and paper coatings and information recording paper.
b) Description of the Prior Art
In recent years, various particulate polymers have been studied for use as additives for coating compositions.
The most commonly used particulate polymer is in the form of homogeneous, emulsion-polymerized polystyrene particles having a diameter of 0.2 to 0.5.mu.. For example, Japanese Patent Laid Open No. 59741/'84 discloses a process wherein an unsaturated carboxylic acid and a vinyl monomer are copolymerized in the presence of an anionic surface active agent and/or a nonionic surface active agent to form a copolymer emulsion in which more than 90% of the particles have a diameter of 0.20 to 0.28.mu.. It is also described therein that this copolymer emulsion can be used in applications including paper coatings, paints and similar applications. However, the organic pigment produced by this process fails to provide sufficient hiding power, brightness and gloss. Accordingly, no practical advantage is obtained unless it is used in large amounts.
In recent years, an organic pigment composed of particles having pores, unlike homogeneous and solid particles, has been proposed in order to further improve hiding power, brightness and gloss (U.S. Pat. No. 4,427,836). This patent discloses a process for producing an aqueous dispersion which comprises providing a dispersion of core material formed from a polymer containing at least 5% of an unsaturated carboxylic acid, adding thereto at least one monoethylenically unsaturated sheath monomer to form a sheath polymer, subjecting this monomer to emulsion polymerization, and neutralizing the resulting emulsion with an aqueous volatile base to swell the core particles and thereby form minute openings therein.
When the organic pigment produced by this process is used in paints or paper coating compositions, an improvement in hiding power and brightness is achieved compared with the organic pigments composed of homogeneous and solid particles. However, gloss is not improved.
Japanese Patent Laid-Open No. 62510/'86 discloses a process for preparing polymer emulsion particles having pores therein, comprising phase separation between the different polymers, and volume shrinkage during polymerization. The organic pigment produced by this process is improved in gloss. However, hiding power and brightness are not improved, because the formed pore is microscopic in size.